Citric acid is produced commercially by fermentation of carbohydrates and the most common source for carbohydrates for such fermentation are beet molasses. It is also known to use glucose or refined sucrose for the fermentation but, while technically this gives very satisfactory results, it has the drawback of being a costlier primary fermentation feed.
For the recovery of citric acid from a fermentation broth it is possible to use either of two types of technologies. In accordance with the first of these, to be referred to herein as the "lime/sulfuric acid process", the fermentation broth is first subjected to so-called "liming", i.e. treatment with calcium hydroxide, the resulting calcium citrate is filtered off, washed, decomposed with aqueous sulfuric acid, the calcium sulfate that forms is filtered off and the resulting aqueous acidic solution is gradually evaporated in a crystallizer whereupon citric acid crystallizes. If desired, the solution of citric acid obtained by decomposition of the calcium citrate with sulfuric acid may be treated with a variety of ion exchangers and adsorbents in order to remove various impurities prior to crystallization.
Citric acid crystals precipitating in the crystallizer tend to be pure and therefore impurities not removed prior to crystallization accumulate in the mother liquor which is saturated and therefore very concentrated in citric acid. Such accumulation of impurities, if not interfered with, inevitably leads to the inclusion of impurities in the crystallizing citric acid and in order to avoid this, crystallization has to be interrupted once a certain concentration of the mother liquor is reached. The then remaining mother liquor, which is enriched in impurities is recycled to the liming operation. Such recycle usually contains 10 to 20% and sometimes even more of the citric acid produced by fermentation, with the consequence that lime and sulfuric acid are consumed in excess over the stoichiometric quantity with respect to the citric acid produced. Furthermore, the recycle also increases the evaporation load on the crystallizer which is due to the withdrawal of citric acid in form of a very concentrated mother liquor and its redilution upon recycle to the much less concentrated starting solution for the crystallization. Consequently the loading of all operations from liming to crystallization on the recycle takes up production capacity which, but for the recycle, could be used for increasing the production of citric acid. Moreover, use of excessive lime and sulfuric acid increases the amount of gypsum that forms in the process and has to be disposed of, which disposal can be quite costly.
These ongoing problems have been known for a long time and there has been a long-felt want for their solution, but so far without success.
The other technology for recovery of citric acid from a fermentation broth to be referred to herein as "direct extraction method", comprises direct extraction with an organic, water-immiscible extractant followed by recovery of citric acid from the extract. Thus, in accordance with U.S. Pat. No. 3,944,606 alkali metal or ammonium citrates are produced directly from a fermentation broth by extraction with a water immiscible organic extractant consisting of a mixture of a secondary or tertiary amine with an organic solvent, and the resulting organic extract is re-extracted with a compound that forms an alkali metal or ammonium salt of citric acid.
U.S. Pat. No. 4,275,234 teaches to extract an aqueous solution of an acid such as a fermentation broth of citric acid with a water-immiscible organic extractant comprising at least one secondary or tertiary amine in which the total number of carbon atoms per molecule is at least 20, dissolved in a water-immiscible, organic, non-polar or polar solvent, and to back extract the resulting organic solvent extract with water at a higher temperature. According to U.S. Pat. No. 4,334,095 a citric acid fermentation broth is extracted with a mixture of a water-immiscible amine and a water-immiscible organic acid dissolved in a suitable water-immiscible solvent, and the resulting extract is back-extracted with water.
The direct extraction method has the drawback that as distinct from citric acid, citrate salts are not extracted from the fermentation broth. Since fermentation broths obtained by fermentation of molasses are as a rule rich with cations the yield of the direct extraction method is, in such a case, rather low. Consequently, as a practical matter, the direct extraction method is useful only where pure carbohydrates are used for fermentation which, however, is not commonly done because of the high costs involved. A further disadvantage of the direct extraction method is the fact that it yields relatively dilute aqueous solutions of citric acid.
It is the object of the present invention to improve the lime/sulfuric acid process for the recovery of citric acid from a fermentation broth so as to decrease the consumption of lime and sulfuric acid and the evaporation load for crystallization and thereby also decrease the amount of gypsum that has to be disposed of, and at the same time increase the capacity of the citric acid recovery from the fermentation broth.
Citrate salts which must satisfy purity requirements as stringent as those of citric acid are conventionally made from crystallized citric acid, crystalline acid being practically the only form of pure citric acid available. This necessarily involves redissolution of the acid. Prior art teaching the production of citrates from citric acid extracted from fermentation broth involves treating large volumes of the dilute broth, which is costly, and (especially for molasses derived fermentation broths) returning the aqueous residue containing unextracted acid and all the impurities to the lime/sulfuric acid process so that the recycle from crystallization is not materially decreased. It is accordingly a further object of the present invention to provide for the complete recovery of citric acid values as pure citrates from small volumes of concentrated mother liquor remaining upon crystallization of citric acid in the liming/sulfuric acid process.